As local energy sources, in particular alternative energy sources such as solar panels and wind turbines, are becoming more widespread in Europe, in the US, and in the rest of the world, the demand for effective and inexpensive technologies for connecting these local energy sources to already existing electrical networks is increasing as well. Many local energy sources, such as solar modules, and also the electrical energy storage devices provide, and in the latter case require, direct current (DC) power. On the other hand, electrical networks, in particular the external electrical grid maintained and administered by an external power provider, work with alternative current (AC) power. In order to convert between DC power and AC power, use is made of power inverters, in the following also just called inverters.
In addition to converting between DC and AC power, modern inverters fulfill a range of other functions. They are able to automatically adjust the output voltage and synchronize with the electrical grid. It is furthermore possible to program them for controlled input and output depending on various electrical and environmental parameters detected through appropriate sensors. Most inverters have to utilize a transformer in one form or the other for creating the appropriate output voltage. This applies especially in cases where a specific inverter technology developed for the European market, where power grids are generally based on the three-phase electric power standard, is modified and adapted for the US market, where power grids are generally based on a split-phase electric power standard. The modification to the inverter then involves utilizing a transformer for converting the three-phase grid connection of the inverter to split-phase standard.
However, the use of transformers makes power inverters heavy and expensive. Transformers can also be an additional source of loss, which can lower the efficiency of an inverter. An example for a transformer-less, unidirectional power inverter is disclosed in US 2011/0299312 A1. This known inverter is a 3-level neutral point clamped (NPC) inverter designed for connecting a solar cell array to a power grid. The DC output of the solar cell array is converted to an AC voltage using pulse width modulation.
It is an objective of the present invention to provide devices and methods for converting between DC power and AC power, which are diversely and flexibly applicable, in particular for use with batteries. They should further be cost effective and have low electrical loss.